Each year, high-profile sportsmen test positive for banned substances. At the same time, new drugs that are not even being tested for come into vogue.

So can the anti-doping agencies ever win - or are they fighting a losing battle against the cheats?

John Brewer, head of human performance at the National Sports Centre in Lilleshall, is not optimistic.

"It's very, very difficult to stay ahead of the cheats," he says. "Perhaps it's a negative view, but all the time the athletes and the shady medical characters who are advising them are always trying to stay one step ahead of the field.

"The testers are closing the gap, but at the same time they are always one step behind," Radcliffe told this website.

"We are getting closer, and it's a big thing that we have EPO testing now. But we don't know how big a problem it is, because we aren't doing the tests."

Michelle Verroken, director of anti-doping at UK Sport, admits that she is involved in a tough fight.

"It is a battle, but not a losing one," she told BBC Sport.

"The fights are not only about drugs, but also in the bureaucracy that surrounds sport, about the lawyers that want to make a smart buck out of this because they can find some smart answer, the athletes who don't want to be tested, officials who don't want to get testing right in their sport

"You see medics making their names and making money from patching up sportsmen and getting them competing again even if that means their careers come to an end prematurely."

Money, money, money

For some it comes down to a simple financial equation: the amount of money a sportsman can make from winning via doping outweighs the amount of money available to those trying to catch them.

"The athletes themselves make money from cheating by being successful, and the medical companies who make the stuff make money from selling the products," explains John Brewer.

"But when you need money for research (into anti-doping), the big countries like the USA effectively block it by not producing their quota of funding."

Verroken, while upbeat, concedes that the struggle is not always an equal one.

"Do we have the resources? We could always do with more," she says.

"I have had postcards from people telling me I'm not catching them - I'm at this event, ha ha, you won't catch me. We get a lot of emails from hotmail accounts. People think they've won and they've cheated the system, but they've cheated themselves."

So how do anti-doping agencies know which sports, individuals or substances to target?

"What tends to happen is emerging practices," explains Verroken.

"You might have identification of certain odd substances in drug tests, and then you ask why they might be there.

"This was exactly the reason why certain substances were picked up in cyclists. It was a treatment for gout - why would healthy athletes be using a treatment for gout?"

The testers are also re-thinking their approaches to solving the drugs menace, as Dr Oliver Rabin of the World Anti-Doping Agency explains.

"We are trying to be as proactive as possible," he says, "creating strong relationships with pharmaceutical companies so we identify a product that has the potential to be a doping substance before it reaches the market.

"It is a new era for us. We use our resources better, we have some very good teams working on the issues and there is much better co-ordination between anti-doping bodies across the world

"There will always be new products being developed, but because we receive the information ahead the marketing of these products, I believe we can win some very significant battles, and hopefully win the final battle."